The case is important in these commands. Also, in some document styles, some of these commands may produce the same size font. For example, if you wanted to just make a small part of your text in a different font, you would use something like:

This is in normal text, while these words are in {\large large text}.

Or, if you wanted to put a larger region in a different size, you'd use something like:

\begin{small}
this will all be in small text
this too.
etc..
\end{small}
This, however, is not in small text